A zinc plated vehicle component or fastening component, such as a bolt, has been enhanced in appearance and corrosion resistance by subjecting to a chromate treatment with a chemical conversion treatment solution containing hexavalent chromium, but according to the tightening of the environmental regulation in recent years, the chemical conversion treatment solution is being transferred to one containing trivalent chromium as a major component. This movement reaches not only the ordinary chromate treatment but also a chemical conversion treatment that is referred to as black chromate with a black appearance, and development and utilization of a chemical conversion product containing trivalent chromium as a major component have been made.
However, black chromate using a chemical conversion treatment solution using trivalent chromium has a problem that desirable corrosion resistance capability and appearance (color tone) may not be obtained, as compared to a chemical conversion treatment using hexavalent chromium, and a solution therefor is demanded. For example, in the black chromate treatment with trivalent chromium, according to the amount of sulfur contained in the chemical conversion film, there is a tendency that the appearance is changed from dark green to greenish black, but the corrosion resistance is rather deteriorated. Furthermore, the final appearance reaches at most black with greenish tone remaining (L value (brightness) of approximately 30), but cannot reach dark black (L value (brightness) of 28 or less), which is achieved by black chromate by hexavalent chromium.
Under the circumstances, in the case where a high antirust treatment is required, it is necessary to use a finishing treatment, such as a top coating, but for retaining the corrosion resistance of the black chromate chemical conversion film as the underlayer, the blackness degree of the finish top coating is necessarily increased for imparting dark black color to the final product since highly dark black color may not be expected by the black chromate chemical conversion film.
However, for retaining the physical property of the film, the coating film-forming composition used in the ordinary top coating cannot contain a large amount of a black pigment, and therefore the top coating treatment is necessarily performed plural times for providing the intended black color, resulting in complication of the process and increase of the cost.